This invention relates to an optical glass having a negative anomalous partial dispersion in the blue region of the spectrum, a refractive index n.sub.d of from 1.73 to 1.83, and an Abbe coefficient v.sub.d of from 31 to 38.
Manufacturers of optical lens systems have for years demanded glasses with which the secondary spectrum of the systems can be greatly suppressed. In particular, it is becoming increasingly desirable to effect correction in the blue region of the visible spectrum. In this region of the spectrum, the relative partial dispersion P.sub.g,F characterizes the optical behavior of the glass.
The dependence of the refractive index on the wavelength of an optical glass is generally characterized by the refractive index n.sub.d at the wavelength 587.56 nm and the Abbe coefficient ##EQU1## where n.sub.F is the refractive index at wavelength 486.13 nm and n.sub.C is the refractive index at wavelength 656.27 nm. The partial dispersion in the blue region of the spectrum is given by the expression ##EQU2## where n.sub.g is the refractive index at wavelength 435.83 nm. The majority of glasses, known as "normal glasses", approximately satisfy the linear relationship P.sub.x,y .apprxeq.a.sub.xy +b.sub.xy .multidot.v.sub.d =P.sub.x,y. The straight line passing through the glasses K7 and F2 (Schott "Optical Glass" Catalogue, Schott Glaswerke Mainz, 1980), i.e., the "normal straight line", is given in the blue region of the spectrum by the equation P.sub.g,F =0.6438-0.001682. v.sub.d. Glasses which differ from this curve have anomalous partial dispersion. The extent of the deviation is characterized by the ordinate different .DELTA.P, by which the relevant P/v.sub.d point has shifted against the normal straight line (Abbe straigt line). Glasses in which .DELTA.P&lt;0, are known as glasses of negative anomalous partial dispersion.
The use of glasses of highly anomalous partial dispersion makes it possible to greatly suppress the secondary spectrum in lens systems for image-forming optics and thus to improve the correction of a lens system of this type.
DE-B 15 96 807 discloses lead/aluminum/boron silicate glasses having refractive indices n.sub.d of from 1.57 to 1.76 and Abbe coefficients v.sub.d of from 29 to 48 with anomalous negative partial dispersion in the blue region. However, due to the high SiO.sub.2 and Al.sub.2 O.sub.3 contents, these glasses have a relatively strong devitrification tendency and require quite high melting temperatures during their production.
German Patent 15 96 856 describes boric acid-free lanthanum glasses in which the problem of poor melting properties is solved by replacing all or some of the SiO.sub.2 and Al.sub.2 O.sub.3 by GeO.sub.2 or Ga.sub.2 O.sub.2 and/or In.sub.2 O.sub.3. However, the advantage of improved melting properties is balanced by the considerable increase in cost of the mixture. Japanese Patent 55, 1481 (B2, 1980) describes glasses having refractive indices n.sub.d of between 1.63 and 1.74, Abbe coefficients v.sub.d of about 32-51 and anomalous negative partial dispersion in the blue region, and containing 5-52% of ZnO and 4-46% of Sb.sub.2 O.sub.3. As a consequence of the toxicity of Sb.sub.2 O.sub.3, these glasses require increased safety measures to be taken for protecting the environment and personnel during production, which thus becomes inconvenient and expensive. Japanese Preliminary Published Specification 53/90311 (1978) describes glasses having n.sub.d of about 1.63 to 1.72 and v.sub.d of about 34 to 41 and containing: 3-11% by weight of SiO.sub.2, 20-35% by weight of B.sub.2 O.sub.3, 10-18% by weight of Al.sub.2 O.sub.3, 40-58% by weight of PbO and 0-5% by weight of each of alkali metal oxides, alkaline earth metal oxides, and La.sub.2 O.sub.3 +ZrO.sub.2 +Nb.sub.2 O.sub.5. The high Al.sub.2 O.sub.3 content, in particular, causes high melting points and a high devitrification tendency.